Although edible oils, such as corn oil, soybean oil, sunflower oil, rape seed oil, etc. are generally interchangeable in cooking applications, corn is significantly different from other oil-bearing seeds when it comes to the recovery of oil from the seed. As opposed to the soybean kernel, the whole of which contains oil, only a small portion of the corn kernel bears oil. Corn germ is the oil-bearing portion of a kernel of corn. This difference affects the rate of extraction of oil from soybean versus from corn kernels. Another difference is that corn germ has a higher oil content than other seeds. Corn germ obtained by wet milling contains 48-52% oil, whereas that obtained by dry milling containing 25-30% oil. In contrast, soybean contains 17-20% oil by weight; rapeseed (canola) contains about 42% oil; cottonseed (29%); sunflower meals (32%), etc.
Solvent extraction is a standard method for recovering oil from oil seeds. However, corn germ cannot be directly extracted with solvent after preparation by flaking, as is done for other oil seeds, because of its tendency to produce a substantial amount of fines which hinder solvent extraction. Consequently, the traditional means of oil liberation from corn germ containing 25-52% oil have resorted to mechanical expression/pressing/expelling. Expelling is conducted to various degrees of oil recovery and may be followed by solvent extraction, for economical reasons.
One conventional process for germ preparation and oil extraction uses dry germ as a starting material. The processing steps include Rehydration, followed by Cooking, and the Full Expelling operation which yields an oil stream. Corn germ is expelled to 6-10% residual oil content which is the lowest oil level economically attainable through direct pressing. In order to enhance oil recovery economics, both germ preparation and germ expelling need to be conducted at high temperatures (250-275.degree. F.). The majority of expelled oil is recirculated in hot and aerated state onto the expeller for external cooling of the barrel, while the quota withdrawn continually is processed hot. As a result, decomposition products leach out of the expeller cake along with the oil recovered. More decomposition products develop further through prolonged handling of hot oil.
In another conventional process for germ preparation and oil extraction, dry germ is Rehydrated, and then Cooked. The cooked corn germ is subsequently expelled to 20-30% residual oil content under similar conditions as in full expelling, discussed in the previous paragraph (example h). This Pre-expelling operation yields an oil stream. The expelled corn germ is again Rehydrated, and then Flaked. In order to enhance oil recovery economics, pre-expelled cake is extracted with solvent to 3-4% residual oil content. This Extraction operation yields another oil stream. Hot solvent extracts more degradation products out of the expeller cake. Consequently, the quality of crude oil (blend of pre-expelled and extracted oils) is inferior to full expelling.
A better process is to use Extrusion as part of the pre-treatment process of corn germ, prior to the oil liberation step. Extrusion has been very effective in improving the solvent extractability of many oleaginous plant materials. Extrusion is well established in the preparation of soybean, rice bran, cottonseed, and pre-pressed canola, sunflower and other oilseeds. However, there are some problems in the extrusion of oleaginous plant materials having a high percentage of oil by weight, e.g., more than 30%. For example, the disclosure of Williams, U.S. Pat. No. 4,901,635, addresses the problems of oil liberation occurring within the extruder as corn germ materials (having more than 30% oil) is processed through the extruder. The liberated oil forms pockets of free oil which squirt out of the dies and interrupt steady-state operation of the extruder. The squirting oil also results in the undesirable loss of oil, the principal product. Williams avoids this problem by providing an extruder apparatus having a means for draining oil liberated from the material during extrusion. A disadvantage with draining out the oil in such a way is that the hot oil will become oxidized. The drainage phenomenon and the oil stream generated thereby are undesirable process elements, because they disrupt process streamlining and make possible oil abuse. Also, it would be advantageous to avoid oil liberation problems through process modifications, so that any conventional extruder could be utilized, which is one of the objects of the present invention.
In a conventional process for germ preparation and oil extraction, like that described in Williams, U.S. Pat. No. 4,901,635, dry germ undergoes several size reduction and conditioning operations at ambient or relatively mild temperature (less than 180.degree. F.), including the steps of Cracking, followed by Conditioning. It is then extruded rapidly at medium temperature (220-230.degree. F.). As a result of extrusion, the oil released remains attached to, or absorbed onto, the porous meal produced through matrix disruption and partial starch gelatinization. Extruded meal is subjected to extraction with solvent to 4-5% residual oil content. The extraction operation yields an oil stream. Due to relatively short exposure time and milder temperature, both meal and oil abuse is limited. Nevertheless, due to extensive size reduction treatment implemented on dry and brittle germ, the process generates substantial quantities of fatty fines. Fines impact on extruded material by making it less porous. Fines retained on the meal reduce extraction efficiency. Additionally, in an air-borne state, fines represent a safety hazard. Greasy fines get into ventilation, which could lead to spontaneous fires.
Accordingly, it is an object of the present invention to provide a non-abusive means of oil release from corn germ.
It is another object of the present invention to enhance oil release from corn germ and to improve oil recovery yield, as well as oil quality.
Another object of the present invention is to rupture oil cells of corn germ to increase solvent extraction rates and improve extraction efficiency.
Another object of the present invention is to reduce fines generated in the process of recovering oil from corn germ, thereby eliminating safety hazards caused by fines.
Another object of the present invention is to reduce capital equipment costs.
Another object of the present invention is to simplify auxiliary operations before and after extrusion as to accomplish a short, streamlined process sequence, which is easier to control than prior known processes.
Another object of the present invention is to achieve enhanced extraction and reduced solvent loss during extraction.
The present invention contemplates a new and improved overall process which provides more efficient processing of a better quality corn oil.
Other objects and advantages of the invention will be apparent from a study of the following specification.
In the present specification and claims, all parts and percentages are by weight, unless otherwise specified.